I would dislike it if Oda took Brooke's abilities in that direction, although it's Oda so I'm sure it would still be good. The ONLY thing that I would like is as you described, him playing a final song with his old crew for Laboon, although that would make the sound dial they made on their death beds(so to speak) kinda pointless, which I would also dislike. So actually, I dislike the idea all together.
Well, apart from that direction, I really don't see how Brooke can get power ups unless his path clashes with Zoro's. The only paths I sorta see as a legit possibility are music and/or haki and/or DF expansion.
I really did not like the idea of a second swordsman joining the crew in the first place. Zoro's going to be the #1 in that department, so Brooke is left to live his life in sux-ville. Unless he takes on an alternate role (bard? summoner? aoe hypnotist xD?)
I suppose the aoe hypnotist part has already come true… though how exactly this will fare in a real fight... who knows.
@ZKaiser like your new pic